1. Field of the Invention
This invention relates to a heat-sensitive imaging material, in particular, a heat-sensitive material suitable as an OHP film with which a reverse image to an original image can be easily formed there on by irradiating the imaging material placed on the original.
This invention also relates to a transparent imaging sheet wherein a stripe of the heat-sensitive composition is coated or attached thereon, either before or after the imaging sheet has been stretched. This stripe is capable of becoming transparent upon the application of heat.
2. Description of the Art
Photothermographic material has long been known as a heat-sensitive material useful for a transparent film, suitable for use with overhead projectors (OHPs), see, e.g., Japanese Patent Kokai Publication No. 52-31728.
One presently used commercial embodiment, Thermofax.TM., from Minnesota Mining and Manufacturing Company (3M,) provides a black image using a combination of a silver salt of an organic acid and a reducing agent, through a reaction of the silver salt. The image is formed in black on a bright background, and an OHP image is projected in black on a white screen. When another color image is desired, a coloring reaction between an electron donating dye and an electron acceptor can be used.
For the formation of the OHP image, a contact exposure method is used in which, infrared (IR) light is irradiated on the heat-sensitive film placed on the original, which causes carbon black therein to absorb heat and heat a part of the heat-sensitive material.
When the projected image is viewed, bright color characters or figures in a dark background not only reduce exhaustion of the eyes, but also make it possible to color the characters or figures by placing a color film on the OHP film.
To create a reverse image on the imaging material film, several products and methods are known, i.e.:
(1) Use of a laminated two-sheet composite consisting of a thermal transfer ink donor sheet and a thermal transfer ink accepter sheet, in which the donor sheet transfers part of the ink to an acceptor sheet, which can be used as an OHP film. See, e.g., Japanese Patent Kokai Publication Nos. 2-118986, 2-128897, 2-128898 and 59-106996.
(2) Infrared copying processing of translucent fusible films which form negative photography due to perforation of the films on the original. See. e.g, Japanese Patent Kokai Publication Nos. 62-51492 and 61-31294).
(3) Film comprising a resin matrix having a low molecular weight organic material dispersed therein, which changes from transparent to opaque when single crystals are converted to a polycrystal. See, e.g., Japanese Patent Kokai Publication Nos. 62-225392 and 63-31790, and
(4) A mixture of an electron donor dye precursor with a developer and a particulate discoloration agent which is coated on a transparent support. See, e.g., Japanese Patent Kokai Publication No. 3-99881.
However, each of (1) to (4) has some disadvantages.
None of the materials exhibit gradation. That is, intermediate tones between the dark and bright of the image cannot be reproduced. This means that such imaging materials cannot be used to copy photographs and the like.
In addition, materials (1) and (2) have low resolution and an upper limit of a heating temperature. Contrast between the bright and dark parts of the image is insufficient.
Transparent sheets are used for imaging through printing or copying devices. Such devices are usually equipped with optical sheet detectors. It is necessary that each transparent sheet has an indicator, such as a white stripe, affixed thereon in order for the sheet to be detected by the optical detectors. Generally, the white stripe is opaque, and thus projects on an overhead projector as a black stripe. When this occurs, image information printed on the transparent sheet in the area of the white stripe will not be visible upon projection and this projected black stripe is also not aesthetically pleasing.
Thus there is a need for a transparent sheet capable of being detected optically in the imaging devices but which becomes transparent after imaging through the devices.
U.S. Pat. No. 5,156,709 discloses a coating composition composed of a polymer pigment combined with an emulsion or latex binder to form a white stripe on a transparency. The polymer pigment particles preferably has a glass transition temperature of between 35.degree.-80.degree. C., and are dispersed homogeneously within a suitable binder such as an emulsion binder. The white stripe becomes transparent upon fusing.
The present invention provides a heat-sensitive imaging material suitable for an OHP film which can easily form a reverse image to an original image, and has gradation and good resolution.
The present invention also provides a transparent sheet having a pattern of a heat-sensitive composition coated or attached thereon, such that the pattern is detectable by any optical detector in an imaging device and is capable of becoming transparent upon the application of heat.